Coupling of a helical conductor to a wave guide



May 5, 1953 F. c. F. o. LUND 2,637,775

COUPLING OF A HELICAL CONDUCTOR TO A WAVE GUIDE Filed March 16, 1948 3nnentor Fltdazlidlmd C(ttomeg Patented May 5, 1953 genus COUPLING OF A HELICAL COND'i JCTOR TO A 'WAVE GUIDE Frederik C. F. 0. Land, Princeton, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application March 16, 1948, Serial No. 15,128

This invention relates t radio frequency coupling devices, and its principal object is to provide improved methods and means for transferring high frequency energy between a helix and a wave guide or transmission line or the like More specifically, it is an object of the present invention to provide improvements in transition or transformer devices for transferring radio en ergy between a coaxial line or a wave guide and a helix with a minimum of reflection.

Another object or" the invention is to provide transition devices of. the described type which opcrate efficiently throughout relatively wide frequency bands.

A further object of this invention is to provide transition devices of substantially radially symmetrical structure, to produce minimum distortion of electric field patterns within the elements which are to be coupled.

The foregoing and other objects will become apparent upon consideration of the following description, with reference to the accompanying drawings, wherein:

Figure 1 is a sectional elevation of a wave guide coupled to a helix in accordance with prior art practice,

Figure 2 is a sectional elevation similar to Figure 1 but illustrating an embodiment of the present invention,

Figure 3 shows application of the transition device of the present invention to coupling a coaxial line to a helix. and

Figure 4 illustrates a modified transition device including separate primary and secondary conductors.

. Transfer of radio energy between a helix and a wave guide or similar device is required in certain types of antenna systems, using helices as radiator or collector elements. Amplifiers of the so-called travelling wave type also include helices which act like electrically long but physically short transmission lines, and must be coupled to wave guides or other enclosed transmission means.

Referring to Figure l, a wave guide l is to be coupled to a helix 3. Only the portion-of the helix 3 in the neighborhood of the wave guide is shown in Figure l. The helix may extend to the right of Figure l to any required length, and may be surrounded by a conductive sheath 5, either throughout its entire length, or only about the portion near the wave guide l, depending upon the purpose for which it is to be used.'

The pitch of the helix 3 is gradually increased in "the direction approaching the wave guide, be

19 Claims. (Cl. 178-44) both Since the effective characteristic impedance of the wave guide may be relatively low compared to the radiation resistance of the conductor '1, some kind of impedance matching device is necessary to effect any substantial transfer of energy. This may be provided by extending the conductor l about one quarter wavelength beyond the wall of the wave guide I, inside a sheath 9 closed by a shorting plug H. The end of the conductor 7 is connected to the plug H. The wave guide I is closed by a conductive plug H2, at a point about one quarter Wavelength from the conductor 1. In practice, both plugs ii and E2 are adjustable in position.

The sheath 9, plug ii, and end portion of the conductor"! form a short circuited coaxial line section, slightly less than one quarter wavelength long to act as an inductance. The end of the wave guide l which is closed by the plug i2 is slightly greater than one quarter wavelength long, and acts as a capacitance in parallel with the inductance of the coaxial line section and the inductance of the conductor 5. The plugs ii and i2 are adjusted to make the inductive and capacitive reactances equal and of such value that substantial impedance match is obtained. The structure of Figure 1 operates eificiently, at the single frequency for which it designed, to transfer energy between the guide 5 and the helix 2. At any other frequency, the matching sections are not the correct lengths and therefore a relatively low impedance is presented to the end ofthe helix. The impedance of the conductor 7! is no longer matched to that of the guide I, and-reflections occur, preventing efficient operation.

Another objectionable feature of the prior artstructure of Figure l. is that the conductor '5 is,

necessarily asymmetrical with respect to the axis of the helx when the only requirement is that of coupling between the wave guide and the helix.

directed along the axis of helix.

which deflects the electron stream away from the axis.

The structure shown in Figure 2 minimizes I the foregoin disadvantageaby using a coupling probe in 'the form of "arelatively large This causes no great d fficulty How-1 ever, in certain applications, for example in. travelling Wave tubes, a stream of electrons is, The Offj center conductor 1 produces a distorted, field] 3 4 diameter cylindrical tube l3 rather than a small a conductor of varying width, similar to the wire. transition section I5 of Figures 2 and 3. The

The transition from the helix 3 to the cylinder secondary helix may be of constant width and I3 is eiiected by a helical section I 5 comprising a is merely an extension of the helix 3. The secconductor which increases in width as the pitch 5 ondary conductor may terminate at a point 27 is increased, so that approximately constant spacwithin the slot l'l formed by the primary helix ing is maintained between successive turns. An- 25. The operation of the device of Figure 4 is other Way of describing this portion of helix 3 is similar to that of the transition section shown that the width of the helix defined as the dimenin Figures 2 and 3, except that no conductive sion (1 (which is the width of the solid portions coupling between the helix and the coupling measured between interturn spaces at right structure is required. angles to the helix angle) increases between suc- The invention has been described as an imcessive turns from right to left as viewed in Fig. provement in coupling devices for the transfer 2. The interturn spaces, measured substantially of energy between a helix and a wave guide or at right angles to the helix angle, as illustrated transmission line, including a transition section by the distances designated 5 in Fig. 2, remains comprising a helical structure of variable pitch. substantially constant. The pitch approaches formed of a conductor having correspondingly infinity as the conductor enters the wave guide a. variable width terminating in a cylinder. The At this point, the conductor forms the holl w term cy r as used r n n n the vcylinder or tube it, with a longitudinal slot 1'5. pended claims is intended to mean the surface The coaxial line impedance matching stub 1.; .y be generated by a line parallel and moving in a omitted, and the cylinder is connected to the. path, not nec ss rily ci u about a directrix wall of the wave guide has shown. and thl S l'iludes shapes of oval. rectangular, n the operation of the structure of Fi ure her s-li s ct n he described struc the cylinder 13 acts like a small antenna or a z; ture has the advantage of providing eii'icient probe to radiate or pick up an electric field coup? ug out a relatively wide band of the wave guide H. The transition s" ises distortion of the e1ecserves to match the characteristic imp. the helix 3 to the radiation resist cylinder l3. The relatively large d the cylinder !3 causes it to h low radiat'on resistance like the h n resistance in Fi u e 1.

e match relatlveiy ch as a coaxial an enclosur J a T625331 g means. the Zescribed strum role, the crossol transition section rectane'ular. The le either by bro-1 and tra-n t to aid in the direction to Figure fiubste y section as that shin n n i to couple a coaxial e to a it is assumed that the outer c conductor whicl .1 pit om that of said helix to infinity at said and increases smoothly in width from that coaxial line is merely an extension of the sheath of the solid portion of said helix to merge sub- 5 surrounding the helix and the inner constantia'lly into the solid walls of said tube.

ductor i9 is of the same diameter as the cylin- 3 he inve tion as S t f rth in claim 2, Whfire? drical element 13. The slot H is short-circuited in said trans tion Se on s coupled 130 said e x by the line conductor l9 at the junction 2!. In by ahelical conductor lying between and insulat the operation of the structure of Figure 3, the ingly spaced from the turns of said transition transition section 15 acts as a transformer to Section and connected to said helix as a continuamatch the characteristic impedance of the line ion thereof.

5, l9 to that of the helix 3. 4.. In a radio frequency system including a Figure 4 shows a modification of the transition helix and transmission means such as a coaxial Section [5 including a secondary helix 23 jormed linef or a waveguide including an enclosure by a conductor lying in the spaces between turns through which radio energy is transmitted, a de- Qf a primary helix 25- Thehelix 25 -0R1Pi 7 iee' iqr treatise-ins energy between said helix and. said transmission means, including a body of metallic material comprising a tube near one endof said helix with its axis coincident with an extension of the axis of said helix, and a transition section connected between said helix andsaid tube, said transition section comprising a generally helical metallic member which increases smoothly in pitch from that of said helix to infinity at said cylinder and increases in the width-of its solid portion from that of said helix to merge substantially into that of said tube, with the spacing between adjacent turns of saidtransition section constant and substantially the same as the spacing between adjacent turns of said helix.

5. In a radio frequency system including a helix of substantially uniform pitch and transmission means such as a coaxial line or a waveguide including an enclosure through which radio energy is transmitted, a device for transferring energy between said helix and said transmission means, including a body of metallic material comprising a tube near one end of said helixwith its axis co-incident with an extension of the axis of said helix, said tube being of substantially the same diameter as said helix, and a transition section connected between said helix and said tube, said transition section comprising a generally helical metallic member which increases smoothly in pitch from that of said helix to a relatively large value at said tube and the solid portion of which increases smoothly in width from that of said helix to merge substantially into that of said tube.

6. A transformer device for radio frequency systems comprising a helix which increases in pitch from a predetermined value at a first end to substantially infinity at the second end, said helix comprising a metallic body which increases from a relatively small width at said first end to a greater width at said second end, whereby said second end merges substantially into a slotted tube having its axis aligned with the axis of said helix.

7. A device for coupling a coaxial line to a traveling wave vacuum tube helix, comprising a transition section in the form of a metallic helix which increases in pitch from a predetermined value at a first end to substantially infinity at the second end, said helix comprising a metallic body which increases from a relatively small width at said first end to a larger width and substantially merging into a slotted metallic tube at said second end the slot in which terminates at said second end, whereby a portion of said second end merges substantially into a solid walled metallic tube, means connecting said solid walled metallic tube to the inner conductor of said coaxial lines, and means coupling said helical transition section to said first mentioned helix.

8. A transformer device for radio frequency systems comprising a helix which increases in pitch from a predetermined value at a first end to a larger value at the second end, said helix comprising a metallic body which increases smoothly from a relatively small width at said first end to a larger width and substantially merging into a tube of radius equal to that of said helix at said second end, whereby the spacing between adjacent turns of said helix is constant and said second end is substantially a slotted metallic tube.

9. A transformer device for radio frequency 1 systems comprising a travelling wave vacuum tube helix which increases in pitch from a predeterminedvalue at-a first end to a larger value at the second end, said helix comprising a me tallic body which increases similarly from a relatively small width at said first end to a width substantially equal to the perimeter of said helix at said. second end, whereby the spacing between adjacent turns of said helix is approximately constant and said second end merges substantially into a cylindrical tube.

10. A transformer device for radio frequency systems including a helix which increases in pitch from a predetermined value at a first end to substantially infinity at the second end, said helix comprising a metallic body which increases from a relatively small width at said first end to a width substantially equal to the perimeter of said helix at said second end, whereby said second end merges substantially into a tube, and a second helix interwound bifilarly with said first mentioned helix and comprising a second metallic body of substantially uniform width spaced insulatingly from said first body."

11. A high frequency system comprising a metallic member, means for projecting through said member the beam of electrons in a travelling wave tube and comprising portions merging successively and smoothly into each other in the order stated, said portions being a tubular portion, a tubular portion with a longitudinal slot, a helical portion with said slot extended into the space between turns and decreasing in pitch, and a helical portion having constant pitch and width.

12. The system claimed in claim 11, said portions being positioned with the axes thereof lying on the same straight line.

13. The members claimed in claim 11, said portions having a substantially constant radius.

14. The combination comprising a travelling wave tube helix through which a beam of electrons is axially projected, a metallic tubular member, and a transition section between said helix and said member comprising an extension of the helix in a continuous spiral having a width increasing with approach to said tubular portion and merging into said tubular portion with a portion of like size and shape having a longitudinal slot, said helix, tubular member and transition section having a continuous axis.

15. A structure for use in a travelling wave tube having a beam of electrons projected along an axis, said structure comprising a metallic member having a closed tubular portion merging into a longitudinally slotted portion which in turn merges into a, helical portion with the slot merging simultaneously into the spaces between the turns of the helical portion, the said helical portion decreasing in pitch and merging into a helix of constant pitch, a tubular sheath surrounding the metallic member and having walls through which the tubular portion merges in contact therewith, said member being insulatingly spaced from said sheath in the other portions thereof and having its axis coincident with that of the electron beam, and a waveguide communicating with said sheath near said tubular portion.

16. In a travelling wave tube having means for producing a beam of electrons. a metallic sheath, a waveguide communicating therewith, a metallic member through which said beam is to be projected, said member being tubular and having one end with a solid side-wall portion extending into a portion with a longitudinal slot beginning substantially at the point of communication of said sheath and said waveguide and said 7. slot extending toward the other end of said memher in a helical spiral having decreasing pitch. said sheath surrounding and insulatingly spaced from said member.

17. A metallic member, means for projecting therethrough the beam of electrons in a travelling wave tube, said member being tubular and having a helical slot increasing in pitch from one end to the other to become a longitudinal slot terminating at said other end with said other. end being a tubular portion with a solid wall.

18. A. structure for a radio frequency energy transition section comprising metallic portions which are helical and longitudinally slotted tubular. said portions smoothly and gradually merging one into the other in the order named, and each having a central axis the continuation of that of the other.

19. A coupling device for the transfer of high frequency energy between a coaxial transmission line and a helix, comprising atubular conductor connected to and forming an extension from end of one of the conductors of said line, said tubular conductor including a helical slot which varies continuously in pitch from substantially infinity at said end of said coaxial line conductor to substantially that of said helix at the other end of said tubular conductor, wherein the width of said slot is substantially uniform and approxh mately equal to the spacing between the turns in said helix.

l0 FREDERIK C. F. O. LUND.

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